1. Field of the Invention
The invention relates to radio frequency identification tags or labels and in particular to a cloaking circuit used to assist in the read operations of RFID transponders.
2. Description of the Prior Art
Marsh et al, "Electronic Identification System," U.S. Pat. No. 5,537,105 (1987) describes an identification system utilizing an interrogator and plurality of transponder tags or labels. The system utilizes an interrogator/reader 2 for identifying one or more transponders 4 within a monitored space as diagrammatically depicted in Marsh's FIG. 1. The interrogator transmits a radio frequency signal, which when received by the transponder, is utilized by the transponder to provide DC power to the transponder. Both the interrogator and transponder utilize coded transmissions. The transponders transmit a reply after a random delay time period allowing the multiple transponders within the same field of view of the interrogator to be individually read due to the reduced chance of collision between respective transponder output signals.
To further improve the readings of the multiple transponders that are simultaneously interrogated, each transponder unit turns itself off after the reader has successfully identified the transponder. The shut down circuit is indicated by a momentary cessation of the interrogation signal. A transponder transmission is disabled by means of flip-flop 38 disposed between code generator 36 and modulator 40. The flip-flop is switched to disable the modulator input responsive to logic circuit 42. Logic circuit 42 monitors the presence of the interrogation signal from the transponder receiving antenna 30. The transponders are subsequently reset by removal of the interrogation signal for predetermined time.
Tervoert et al, "Electromagnetic Identification System for Identifying a Plurality of Coded Responders Simultaneously Present in an Interrogation Field," U.S. Pat. No. 5,124,699 (1992) describes a system which, like the Marsh system, utilizes a plurality of RF field power transponder 40 responding to an interrogation signal after a random delay with an encoded transmission. Upon successful identification of the transponder, the transponder switches to a passive mode and remains in such mode until reset by removal of the interrogation signals or a change in its frequency.
Pilested, "Security System for Survailing the Passage of Commodities Through Defined Zones," U.S. Pat. No. 5,471,196 (1995) shows in FIG. 4 a transponder including a transmitter 5 and receiver 4 which are coupled to an antenna 7. Antenna 7 is tuned by capacitor 1 through a switch 3. Switch 3, receiver 4 and transmitter 5 are controlled by logic circuit 6. Subsequent to the transmission period, T1, switch 3 is opened detuning antenna 7 and preventing transmission through antenna 7 until the next transmit time period. During time period T2 that switch 3 is open, the security system is able to receive reply signals from other transponders.
Jeuch et al, "Automatic Identification System for Objects or Persons by Remote Interrogation," U.S. Pat. No. 5,528,221 (1996) describes a system of RF field power tags which are interrogated by a coded transmission. The tags respond and upon successful receipt of the tag's response, the tag inhibits itself utilizing one of flip-flops 26 as shown in FIG. 3 to temporarily inhibit further operation.
Evereft et al, "Identification System and Method With Passive Tag," U.S. Pat. No. 5,492,468 (1996) describes a system having a portable tag 14 which includes a voltage doubler 42 for providing a DC voltage to a storage capacitor 44. When the voltage across capacitor 44 exceeds 5 volts, a power-up circuit 48 closes switch 50 to provide power to remaining tag circuits 38. This in turn causes capacitor 44 to be discharged. Capacitor 44 has sufficient charge on it to insure that the tag can completely transmit its coded information. As capacitor 44 recharges, switch 50 is maintained open by a time-out circuit, which limits how often the tag can provide a transmission, thereby enabling the identification system to receive signals from other tags.
Cato et al., "Time Division Multiplexed Batch Mode Item Identification System," U.S. Pat. No. 5,539,394 (1996) describes a system for reading a plurality of identification tags and labels in which a interrogator/reader broadcasts an interrogation signal which initiates transmission from the tags. The tags are designed to output within a predefined time slot. Subsequent to successful communication with the reader, an acknowledge signal is transmitted to the tags. The tags cease communication in response to the acknowledge signal thereby allowing similarly coded tags to be read. Where collisions occur, the interfering tag retransmits signals in different time slots based on the timing signal transmitted from the interrogating system.
Dingwall et al., "System and Method for Remote Identification of Coded Articles and the Like," U.S. Pat. No. 5,502,445 (1996) describe a system in which badges 14 are interrogated by a beam 16 transmitted from directional antennas 18 of an interrogator/receiver unit 12. Dingwall was cited for showing that once a badge has been identified, an electronic circuit is put into an inactive or power-down state so that it will no longer respond to the interrogation/receiver unit as long as the badge remains within the range of beam 16 in order to facilitate identification of other badges 14.
As illustrated above, the primary prior art method for preventing RFID transponders from interfering with on-going reader operations, once their data has been read, has been to use a logic circuit to disable the RFID transponder from replying to the interrogation signal, the purpose being to improve communication with multiple tags by reducing the likelihood of interference. Typically, upon successfully transmitting the data, the tag receives a coded signal from the transmitter acknowledging its receipt. Logic circuits on the tag then enter a state that prevents further responses by the tag until either a second coded signal reactivates a response mode or until the tag is removed from and then re-enters the powering RF field. The powering off and on cycle has the effect of resetting the logic circuit in the RFID tag allowing it to respond anew.
It is also known to detune the tag's antenna by at least partially shorting out the antenna, as either a means of signaling or for depleting the energy stored in the antenna resonant structure.